1. Field of the Invention
The invention relates generally to the field of transmit-receive laser-detector systems, and specifically to spectrometers that are frequency agile as to a given set of spectral lines.
2. Description of Related Art
Measuring the relative atmospheric plus target absorption of a set of spectral lines by a remote sensor or detector can be done sequentially (serially) or simultaneously (in parallel). The parallel approach is preferred in that transmitting the desired wavelengths simultaneously insures that the atmospheric turbulence is the same for all the wavelengths. If the sequential approach is used, the variation in atmospheric turbulence will cause the return signals to vary, which could be misinterpreted as a wavelength dependent absorption.
One typically wishes to transmit m wavelengths out of an available spectrum of n wavelengths. The returned signals at each of the m wavelengths must be individually measured by the receiver. The system must then be tuned to transmit and receive a different set of m wavelengths out of the available spectrum of n wavelengths. As a specific example, a remote chemical sensor may use CO.sub.2 lasers to transmit and receive four (m=4) different CO.sub.2 spectral lines in order to measure the relative atmospheric plus target absorption at the four wavelengths. At another point in time, a different set of four lines may be transmitted and the relative absorptions measured again. The subset of spectral lines transmitted may be selected from a set of approximately 70 (n=70) possible known and otherwise available CO.sub.2 spectral lines in the spectrographic region from 9 to 11 microns with spectral separations between selected lines ranging anywhere from about 0.02 microns to 2 microns.
In the parallel mode, different embodiments have been suggested or used in the past. One prior art embodiment concerns the case where if only two spectral lines are concurrently in use at any given time (m=2), then polarization sharing of the transmitter area and polarization splitting of the received beams could be utilized, but this has the potential of introducing a systematic error into the measurement if the target reflectivity or atmospheric transmission is polarization sensitive, which generally is the case. It will be also noted that there is usually a large degree of, if not total, depolarization by the target, thereby precluding received signal separations.
In the transmittal mode, different parts of the aperture or different apertures for the different spectral lines could conceivably be used. Unfortunately, this acts to waste transmitter aperture by a factor of m for m spectral lines. For the received beam, all spectral lines necessarily use all parts of the aperture, thereby precluding use of this method for the receiver.
Another prior art receiver mode embodiment was to use a spectrometer to disperse the spectrum of the received signals and dedicate one detector element to each of the possible spectral lines, whether present or not, that is n detectors for n possible spectral lines, of which only m are used at any one time. This is relatively impractical particularly in regards to CO.sub.2 lasers, not so much because of the number of detectors, but because of the range of wavelengths and the line-to-line spectral separation dictates a detector array about three centimeters long laid out along a curved focal plane. This array must then be cryogenically cooled and cold shielded, although the cold shielding will be relatively inefficient because it must be totally open in the direction of the array. The detectors are not equally spaced on the array, but must be spaced according to the exact wavelengths of the spectral lines and the diffraction grating dispersion equation. In addition, the large number of detectors on the curved focal plane means that several arrays of detectors rather than a few individually mounted detectors must be used, and the quality of each detector is not as good as with individual mounted detectors because each array must be either wholly accepted or rejected. It will also be appreciated that an additional problem concerns the need for n preamplifiers for n possible lines or using low signal level switching, and a method of getting all of the leads out of the package. The analogous transmitter solution would be to have n lasers for n possible lines, only m of which are used at any one time, which would have obvious disadvantages.
It will be appreciated that in a receiver system, one would like to utilize no more detectors than spectral lines transmitted and also have minimal loss of signal in the optics thereof. One way to accomplish this is to disperse the received signal and move a limited set of detectors to the focal spots of the constituent spectral lines. Under the prior art, the spectrum of the received energy can be dispersed with a diffraction grating, and the focal spots of the received spectral lines may be separated by spatial distances ranging from 1/2 mm to 4 cm for the CO.sub.2 system, depending on exactly what combination of lines is transmitted. The linear dimensions can be magnified or demagnified by choice of optics focal lengths, but the range from farthest to closest spacings will remain at 40:1. At any given time, some or all of the spectral lines required may be adjacent, requiring the detectors to be separated by successive 1/2 mm spacings, or at any given time some lines may be widely separated, requiring the relevant detectors to be separated by 4 cm. Unless each of the detectors and its cryogenic cooler is only 1/2 mm in diameter, it is impossible to move the detectors to the appropriate locations. Even if the detectors could be so moved, or if a set of fixed detectors were used with articulated optical trains, the large, precise mechanical motions required would dictate a very complicated system. The large, mechanical motions would also drastically lengthen the time required to switch between sets of spectral lines. An analagous mechanical motion problem occurs with a transmitter system that combines m spectral lines from m sources into a common aperture by implementing a dispersive system backwards.
Yet another prior art embodiment is to use beamsplitters to break the received beam up into a number of beams equal to the number of spectral lines transmitted, and then use a diffraction grating and a single detector for each of the split-off beams. Each diffraction grating directs one of the known spectral lines onto its detector, and thus the rest of the energy in each split-off beam is lost. It will be appreciated that this is very inefficient in that an m-spectral line system suffers a loss of a factor of m.
The analogous transmitter solution would be to overlay the transmitters using beamsplitters, but this also throws away a factor of m in transmitter energy for a m-laser system.
What is needed is an invention that uses only m lasers and m detectors. The lasers are internally tuned to the desired set of m spectral lines and diffraction gratings are used to overlay the m laser beams onto a common output aperture. On the receiver end, a diffraction grating separates the m spectral lines and directs the m signals to only m detectors. This invention provides a practical and compact method of accomplishing both the transmit and receive functions.